As the prior art, Korean Patent Application No. 2001-88387 discloses an invention entitled “Method and apparatus for PAPR reduction using soft-clipping method in OFDM wireless communication system”
In the patent, the apparatus reduces the PAPR by soft-clipping a signal of an inverse fast Fourier transform output, in which the clipping operation is not performed when the signal is less than a soft-clipping starting point and the clipping operation is performed by a disclosed equation when the signal is between the soft-clipping starting point and a soft-clipping ending point. At this time, a multiplier and a divider are required. Accordingly, the PAPR is reduced by clipping the signal without signal distortion so as to prevent distortion in a band and deterioration caused by an out-of-band clipping noise.
In addition, PCT Publication No. Wo03/041327 (filed Nov. 9, 2002) discloses an invention entitled “Apparatus and method for reducing PAPR in an OFDM mobile communication system”.
In the OFDM mobile communication system, a transmitter pre-emphasizes an output signal of an IFFT block, and a receiver de-emphasizes a received signal before FFT, thereby efficiently reducing PAPR due to multiple subcarriers. When the output signal of the IFFT block is greater or less than a predetermined value, the signal is determined by transmission functions respectively multiplied by different gains. That is, when the transmission signal S(n) is converted to a signal Spre(n) through a transmission function of pre-emphasis, the receiver performs de-emphasis for converting the received signal Spre(n) to the signal S(n). Accordingly, the apparatus is formed by a digital signal process (DSP) since it is required to perform many calculations.
In addition, a paper entitled “PAPR reduction using soft clipping and ACI rejection in OFDM system” has been published in IEEE Transaction on Consumer Electronics, vol. 48, pp. 17-22, February 2002
In the paper, the soft clipping method rather than a hard clipping is suggested to reduce the PAPR of the OFDM transmission signal, in which the soft clipping is performed in a like manner of the above patent. That is, the soft clipping is performed between the soft-clipping starting and ending points. A filtering operation using additional FFT and IFFT processes to remove clipping noise is performed since the clipping causes clipping noise in the transmission signal. The filtering operation removes the out-of-band noise, but the noise in the band still exists. IFFT and FFT units are additionally provided so as to filter the clipped transmission signal.
In a method for clipping a complex number transmission signal to reduce the PAPR of the transmission signal in the OFDMA system, the clipping operation is performed without any phase error by dividing real and imaginary parts by a calculated magnitude of a complex number transmission signal.
However, it is complicated since the multiplier and the divider operate for every sample, and therefore signals of the real and imaginary parts are separately clipped, allowing a phase error in the method. The method is ideal for clipping signals, but a hardware configuration therefor may be complicated.
In another method for clipping the complex number transmission signal, while the signals of the real and imaginary parts are separately clipped without an additional multiplier and divider, there is a problem in that the phase error is allowed and the PAPR on the same clipping level is still great.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.